The most common reference element is a silver (Ag) wire coated with solid silver chloride (AgCl) and immersed in a reference electrolyte
consisting of a potassium chloride (KCl) solution. Maybe you wonder why
silver? Silver is the best electrical conductor of all metals. And when
we say best, we mean with the lowest electrical resistance.
The solid silver chloride element at the end of the silver wire forms the transition to the KCl-electrolyte.
The half-reaction that occurs is:
The chloride ions (Cl-) are the carriers of the electrical charge between the silver wire and the reference electrolyte.
Another advantage is that the silver chloride reference is relative insensitivity to changes in temperature, which is mainly dependent on the chloride concentration of the reference electrolyte.
The silver chloride element has to achieve two major objectives.
Firstly, to connect the electrolyte to the connecting cable, and
secondly to provide stable electrical reference point for the voltage measurement.
chloride is prone to chemical reactions, especially with sulfides.
Solutions containing sulfide form silver compounds in the diaphragm that
are difficult to dissolve.
Silver sulfide contamination can easily be identified by a blackened diaphragm, and a greatly increased diaphragm resistance leading to a substantial increase in the response time and to degradation of the pH measurement. Furthermore, such a contaminated electrode is often impossible to calibrate.
Read what to do to prevent silver chloride precipitation.
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